1. Field of the Invention
This invention relates to temperature indicating paint and a method of preparing a specimen with the same.
2. Description of the Related Art
Coatings which change color or some other physical property at one or more known temperatures are a useful tool in the development of aero-engine components. Techniques which employ such thermal coatings have several advantages over other methods in that they are non-destructive, relatively low cost, and produce a temperature profile over the whole surface of a component, rather than just at discrete points as with thermocouples.
Temperature indicating paints, or thermal paints as they are also known, have shortcomings, one of which is that the color/physical changes are, to a varying degree, time dependent and to a lesser extent pressure dependent. The usual technique to overcome this problem is to temperature calibrate all multichange paints (which when subjected to a thermal gradient may reveal several permanent changes, each being assigned a temperature isotherm) at various times in the laboratory. However, temperature is not the only parameter affecting these changes, as other environmental conditions may also play a part.
To overcome the time dependent nature of known thermal paints, a datum coating can be applied to provide an isotherm on a component, under operating conditions, which has been previously demonstrated to be time and environmentally independent. The mechanism involved in the change to form the isotherm should be a function of temperature, and irrespective of the operating gas environment so that a datum isotherm can be allocated to the component with confidence regardless of running time or operating conditions. Ideally, at least three datum markers covering a suitable temperature range should be used for internal calibration of a multi-change thermal paint.
Metal coatings have previously been used for internal datum markers utilizing silver, gold and gold/silver alloy. Gold and silver have been applied by electro-plating and gold/silver alloy by flame spraying. However, with particular reference to aero-engine development with components such as turbine blades, such coatings have been found unsatisfactory. The methods of application quoted above usually result in an average coating thickness of 25 microns to 30 microns for electro-plating and 50 microns to 75 microns for flame spraying. This amount of material has been known to cause problems when flowing in the molten state by blocking turbine blade cooling holes. These processes are also relatively costly and time consuming whilst requiring expensive equipment. When plating turbine blades with gold and silver, it is difficult adequately to mask off the leading and trailing edge cooling holes prior to plating. These masking difficulties result in silver and gold plate being missing from crucial areas and hence gaps will result in the data necessary to assess the blade temperature profile. Furthermore, the surface of a flame sprayed coating is extremely rough, causing both contamination and interpretation problems.